ozoneoceantrevoramuellerThe point of that is to draw the characters naked without actually having them naked :)
Back to comic sexuality, what about characters who wear “skin tight” outfits?
Why make them “naked” in that way? Well, tradition is one reason. Superheroes are direct descendants from Greek heroes from Myth. And then o fcourse there's the continuing tradition within the culture itself. You draw them that way because everyone's always drawn them that way: that's how you draw them, that's how they look!
Whether super heroes are modern “myths” or not is a whole separate area for debate. There are many who argue that super hero stories fullfil none of the cultural functions of a myth and are, therefore, no more than escapist entertainment. Personally, I think you can argue either case. In fact, Chapter 4 of Shades specifically has a line which posits that, at least in some respects, they most definitely are. (No don't go looking for it now - Chapter 4 isn't online at DD yet!) Either way, I don't believe that's a large part of the reason why they tend to be drawn effectively naked.
I believe the real reason lies in the “everyone's always drawn them that way” argument. Siegel and Shuster designed Superman's costume around the skin-tight outfits worn by the circus strong men of their day and that's been the template ever since. It's become one of the accepted “norms” of the genre.